1. Field of the invention
The invention relates to an oblique mirror-type normal-incidence collector system, in particular for light sources which predominantly emit radiation with wavelengths of ≦193 nm. Most particularly preferred are EUV light sources, in particular EUV plasma discharge sources. In addition, the invention describes an illumination system comprising such a collector system, which preferably is assigned to an EUV projection exposure unit, in particular for EUV lithography.
2. Description of the Prior Art
In order to be able to still further reduce pattern widths for electronic components, in particular to the submicron range, it is necessary to reduce the wavelengths of the light used for microlithography. The use of light with wavelengths of ≦193 nm is conceivable, for example, lithography with soft x-rays, so-called EUV lithography.
EUV lithography is one of the most promising lithography techniques for the future. At the present time, wavelengths in the range of 11-14 nm, in particular, 13.5 nm, are discussed as wavelengths for EUV lithography, with a numerical aperture of 0.2-0.3. The image quality in EUV lithography is determined, on the one hand, by the projection objective, and, on the other hand, by the illumination system. The illumination system will provide an illumination that is as uniform as possible of the field plane, in which the pattern-bearing mask, the so-called reticle, is disposed. The projection objective images the field plane into an image plane, the so-called wafer plane, in which a light-sensitive object is disposed. Projection exposure systems for EUV lithography are designed with reflective optical elements. The shape of the field of an EUV projection exposure unit is typically that of an annular field with a high aspect ratio of 2 mm (width)×22-26 mm (arc length). The projection systems are usually operated in scanning mode, wherein the reticle will be moved in the field plane and the light-sensitive object, typically a wafer with a suitable photoresist, will be moved in the image plane, synchronously relative to one another. With respect to EUV projection exposure units, reference is made to the following publications:    W. Ulrich, S. Beiersdörfer, H. J. Mann, “Trends in Optical Design of Projection Lenses for UV- and EUV-Lithography” in Soft-X-Ray and EUV Imaging Systems, W. M. Kaiser, R. H. Stulen (editors), Proceedings of SPIE, Vol. 4146 (2000), pp. 13-24 and    M. Antoni, W. Singer, J. Schultz, J. Wangler, I. Escudero-Sanz, B. Kruizinga, “Illumination Optics Design for EUV-Lithography” in Soft X Ray and EUV Imaging Systems, W. M. Kaiser, R. H. Stulen (editors), Proceedings of SPIE, Vol. 4146 (2000), pp. 25-34the disclosure content of which is incorporated to the full extent in the present Application.
For uptake of the radiation of EUV light sources, in particular of laser-plasma sources and of discharge sources, grazing-incidence collectors are utilized according to the prior art, i.e, those in which the EUV radiation strikes the reflective surfaces at a grazing incidence with the formation of a total reflection. Such collectors can be designed, for example, as nested systems, which are comprised of several collector shells and at which two reflections occur each time. Such nested collectors are also called Wolter systems. Mirror systems are utilized in such collector systems, these mirror systems, for example, consisting of a combination of hyperboloid-shaped and ellipsoid-shaped mirrors and whose principle was described for the first time in the literature in Annalen der Physik 10, 94-114, 1952, wherein the disclosure content of this document is incorporated to the full extent in the present Application.
The advantage of this type of collector consists of the fact that even with large-scale light sources, such as, for example, discharge sources, the power irradiated in the half-space is taken up and bundled in the forward direction. A corresponding design is known from U.S. Pat. No. 5,763,930. In this case it also must be taken into consideration that it is not possible using this principle to bend back the beam path in the vicinity of the light source which would reduce the size of the illumination system, and also the extension of the light source also runs counter to such bending back according to the current prior art.
Another disadvantage of grazing-incidence collectors is their shadowing effects, which arise due to the unavoidable mechanical holders, particularly in the case of a nested structure. These collectors can be constructed in a filigree pattern, e.g., in the form of a spoked wheel, but these lead to radiation losses especially in the case of small sources. A similar problem results with respect to the cooling equipment of the grazing-incidence collector, which adds further mechanical structures and thus contributes to the shading losses. Such cooling equipment cannot be omitted, since for lithography at 13.5 nm, due to the necessity of forming a vacuum, the heat transfer is insufficient and the thermal stress would lead to an intolerable deformation of the mirror shells.
Collectors with a single normal-incidence collector mirror, such as have become known, for example, from EP 1,255,163, can in fact be well cooled on the back side, but a shading which results from the finite extent of the light source cannot be avoided. If a discharge source is present instead of a laser plasma source, then the space requirement which results due to the extended electrodes leads to an intense reduction of the illumination power. In order to avoid this problem, another bending back can be undertaken in the vicinity of the source. Such collectors, which are composed of two normal-incidence mirrors disposed in the form of a Schwarzschild system, have become known from U.S. Pat. No. 5,737,137 and have also been propsed in EP 1,319,988 A2 for the individual focusing of EUV light sources in an arrangement with a plurality of light sources. Here, it is impossible, for reasons of space, to arrange a particle filter after the light source, so that there is a particularly rapid polluting of normal-incidence mirrors in the vicinity of the source. In addition, the collectors with Schwarzschild arrangement, which are known from the prior art, are characterized in that the radiation emitted from the EUV light source is taken up by the first normal-incidence collector mirror with a small numerical aperture of typically NA˜0.3. When going to larger collection angles in such a system, the angles of incidence on the second collector mirror increase in relation to the surface normal line, which is accompanied by a reduced reflectivity and an increased polarizing effect of the multi-layer reflection coating.